This invention is generally directed to processes for the preparation of toner compositions, and more specifically semi-suspension polymerization processes. In one specific embodiment, the present invention is directed to semi-suspension polymerization processes for the economical preparation of toners with, for example, an average diameter of from about 5 to about 25 microns, wherein a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component are bulk polymerized until partial polymerization is accomplished, for example from about 10 to about 40 percent of monomer or comonomers is converted to a polymer, followed by adding to the formed partially polymerized polymer pigments, or dyes optional additives such as charge control materials, low molecular weight waxes such as polypropylene, or polyethylene, and the like, and mixing with, for example, a high shear mixer to obtain a homogeneous organic mixture, subsequently dispersing the resulting organic mixture in water containing a stabilizing component, which dispersing can be accomplished, for example, by a high shear mixer, transferring the resulting suspension to a reactor, and effecting polymerization thereof, followed by optional washing, and drying. The toner composition obtained can be optionally blended with surface additives, which may function as flow aids, such as colloidal silicas and the like.
Toners have been prepared generally by fusion mixing of pigments (colorants), charge control agents and other additives into thermoplastic resins to disperse them uniformly therein. In view of the high viscosity of the mixture, a considerable amount of energy is needed to achieve uniform dispersion of pigments and other additives in the toner resin. The mixture is then cooled, followed by pulverization and classification into desired particle sizes and particle size distribution. It is well known that pulverization is an energy intensive step in this process. This preparation method is capable of producing considerably excellent toners, but requires the use of a several steps which are costly, energy intensive and are limited in certain respects. In the process for producing toners by pulverization, the material must be fragile so as to be readily pulverized to a certain extent. Therefore, some thermoplastic resins which are not fragile but have good fusing performance cannot be selected for the aforementioned prior art process. However, if the material is too fragile, it may be excessively micropulverized and, therefore, the fines portion of the particles must be uneconomically removed. These limitations become increasingly severe for smaller particle size toners. When a material with a low melting point is employed to improve fusing performance of the toner, fusion of such material may occur in the pulverizing device or the classifier.
Accordingly, in order to remove the drawbacks of the pulverization method, there have been proposed processes for producing toner wherein the toner particles were formed and correct particle size distribution produced in a reactor. These processes include dispersion polymerization, suspension polymerization, emulsion polymerization, and the like. Disclosed in U.S. Pat. No. 4,486,559 is the preparation of a toner composition by the incorporation of a prepolymer into a monomer/pigment mixture, followed by emulsion polymerization, see for example columns 4,5 and 8 of this patent. Also, methods of preparing toner, including suspension/dispersion polymerization, are detailed in columns 1 and 2 of this patent. In these processes, the pigment and additives such as charge control components are added to a monomer, or comonomers prior to polymerization. Particle formation is achieved by the dispersion of the pigmented monomer, or comonomers in a continuous phase such as water, and the droplets of pigmented monomers are then polymerized to form toner particles. One advantage of these processes as compared to many other methods is the elimination of fusion mixing (Banbury/extruder) and pulverization classification processing. Nevertheless, it is difficult with these processes to accomplish polymerization of pigmented monomer droplets in a diameter range of 5 to 25 microns with a narrow distribution of particle diameter of, for example, 1.3. Also, suspension failure is common with these processes especially when monomer droplet diameter is less than 50 microns and as polymerization proceeds in the sticky region (10 to 40 percent conversion). Further, it is difficult to conduct the polymerization of pigmented monomer droplets since, for example, it is well known that polymerization of free radical type monomers are hindered, and many times prevented by the presence of various pigments, especially carbon black. Another disadvantage of these processes for the preparation of toners resides in the resulting poor dispersion of the pigment and other additives within particles which is believed to be caused by the lack of micromixing, that is the mixing within monomer droplet. Moreover, the prior art processess for the preparation of particles is costly, or not as economical as desired. These and other disadvantages are avoided or minimized with the semi-suspension polymerized toner process of the present invention.
As a result of a patentability search there were located U.S. Pat. No. 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization; U.S. Pat. No. 4,680,200 and U.S. Pat. No. 4,702,988, which illustrate emulsion polymerization.
Disclosed in copending application U.S. Ser. No. 341,454 filed Apr. 21, 1989 is a process for the preparation of polymeric particles which comprises mixing at least one monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until from about 10 about 50 weight percent of the monomer has been polymerized; dispersing with a high shear mixer the aforementioned partially polymerized product in water containing a stabilizing component selected from a group consisting of non-ionic and ionic water soluble polymeric stabilizers to obtain a suspension of particles with an average diameter of from about 0.1 to about 5 microns in water; and polymerizing the resulting suspension wherein bulk and suspension polymerization is accomplished by heating at a temperature of from about 30.degree. C. to about 200.degree. C.